Caustic resistant coatings



Patented July 29, 1947 CAUSTIC RESISTANT COATINGS Paul J. Gegner, Barberton, Ohio, assignor to Pittsburgh Plate Glass County,

Company, Allegheny Pa., a corporation of Pennsylvania No Drawing. Application October 27, 1944, Serial No. 560,713

The present invention relates to protective films for metal containers and more particularly to a caustic resistant coating for containers and other apparatus employed in connection with the manufacture, storage, shipment or use of concentrated solutions of the hydroxides of the alkali metals.

One object of my invention is to provide a composite coating which adheres strongly to the surface of metal used in the fabrication of apparatus for storing or handling concentrated caustic solutions and which is highly resistant to the corrosive action of the caustic solutions.

A second object of the invention is to provide a composite coating for metal articles which is readily applied thereto by the simple methods well known to the paint and lacquer industries.

Other objects and advantages of my invention will be apparent from the following detailed description of a preferred embodiment thereof,

Alkali metal hydroxides are valuable reagents in the manufacture of soap and paper, in refining oils, in bleaching and in dyeing. Ordinary processes of manufacture result in solutions having an hydroxide content of from 8 to 12 percent by weight which, for most applications, are so dilute as to necessitate concentration. Furthermore, it is desirable to concentrate these solutions in order to reduce the shipping cost incident to their transportation from place of manufacture to.

place of use. Evaporation to approximately 50 percent concentration is readily effected and for many purposes caustic of this concentration is acceptable.

Recently developed methods have enabled the preparation of pure caustic solutions of as high as 80 percent concentration. Obviously, this further reduction in water content occasions even greater savings in cost of transportation. Alkali hydroxides of these high concentrations are solids at ordinary temperatures and to facilitate handling, they are loaded into tank cars or other containers at a temperature of approximately 275 C. The concentrated caustic is extremely corrosive at ordinary temperatures and when heated it will attack the metal containers to an even greater degree. This attack or corrosive action is objectionable not only from its eifect upon the containers but also because the metal pick-up or contamination of the caustic itself. a

2 Claims. 117-75) The application of caustic resistant linings to metal containers has been investigated in the past. Heretofore, coatings or linings of nickel have been suggested and although they are successful for the intended purpose, the cost is prohibited. Cellulose ethers alone or modified with various synthetic resins have been employed with considerable success. Vulcanized films of rubber have also been used to form caustic resistant coatings. The coatings heretofor employed have not, however, proved universally suitable for the protection of metal containers intended for transportation and storage of concentrated caustic.

Briefly stated, the present invention contemplates a composite coating embodying a vulcanized polychloroprene film adhered to a metal sur face by means of an intermediate or priming film consisting of vulcanized after-chlorinated polychloroprene which contains from to 62 percent. of chlorine.

' In the practice of my invention, the metallic surface to be protected is grit-blasted in order to clean and roughen it. The cleaned surface is thereupon treated with two layers of after-chlorinated polychloroprene containing from 55 to 62 percent of chlorine admixed with suitable vulcanizing agents in a mutual solvent. These primer coats are each air-dried for one hour or for such time as is required for them to become dry to the touch. Thereafter, eight or more sprayed coats of compounded polychloroprene latex are applied at the rate of approximately one coat an hour. The coated metal is then dried for 24 to 48 hours, depending upon the relative humidity, at F. Finally, the container is loaded with 73 percent caustic and held at 240 F; to effect vulcanization, not only of the surface coats of compounded polychloroprene but also of the primer coats of after-chlorinated polychloroprene.

The following constitutes a specific example of coating compositions used in the application of a caustic resistant lining to a tank car:

Primer coat.-l00 parts of after-chlorinated polychloroprene containing 62 percent of chlorine was dissolved in xylol and compounded with 10 parts of carbon black and 5 parts of zinc oxide. The carbon black does not form an essential portion of the composition but is added solely for its color.

Finish coat-The compounded polychloroprene employed for the final coatings has the following composition:

Agerite is an antioxidant comprising phenylbeta-naphthylamine, while Aquarex D consists principally of the one-half sodium sulfate ester A of a mixture of the higher fatty alcohols.

The coating composition is compounded by Preparing a ball mill dispersion of the water insoluble ingredients and then mixing this dispersion with the polychloroprene latex.

Two primer coatings were applied and airdried and 12 coats of the compounded polychloroprene applied at the rate of one an hour. The film was air-dried at room temperature for 24 hours and then cured under 73 percent caustic at 240 F. Th cured film had excellent adhesion to the met 1 under the ordinary conditions of use and, in fact, was unaffected when exposed to temperatures as high as 300 F.

When the experiment was repeated with the substitution of chlorinated polychloroprene containing 47 percent of chlorine in the primer coat, good adhesion was obtained initially but it was not satisfactory under conditions of test. Similarly, the employment of chlorinated polychloroprene containing 67 percent of chlorine in the primer coat permitted the formation of a liner which was of excellent appearance at first, but which failed rather quickly under test. As a result of these tests, it is believed that only chlorinated polychloroprene containing from 55 to 62 percent of chlorine may be employed successfully in the practice of my invention.

It will at once be apparent that various modifications in the several compositions and the manner of their application are possible without departing from the spirit of the invention and the scope of the appended claims.

I claim:

1. A method of providing a metallic container with a liner which is resistant to the corrosive action of 73 per cent caustic at 275 E, which method comprises applying a Primer coating of chlorinated polychloroprene containing from 60 to 62 percent of chlorine and having admixed therewith approximately 5 percent by weight of zinc oxide, applying a plurality of coatings of compounded polychloroprene latex over the primer coating, air-drying the composite film so formed and thereafter vulcanizing the several elements of the film under 73 percent caustic at 240 F.

2. A method of providing a caustis resistant lining for a metallic container, which lining will Withstand the corrosive action of 73 percent caustic at temperatures in excess of 250 R, which method comprises subjecting the metal surface to a grit-blast for cleaning and roughening, applying to the cleaned metal surface a priming coat of chlorinated polychloroprene containing 62 per cent of chlorine ,and compounded with approximately 5 per cent by weight of zinc oxide as a vulcanizing agent, air-drying the priming coat, superposing upon the priming coat a plurality of layers of polychloroprene latex containing zinc oxide as a vulcanizing agent, air-drying the surface coatings and thereafter subjecting the composite film to vulcanization under 73 percent caustic at 240 F.

PAUL J. GEGNER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,067,172 Carothers Jan. 12, 1937 2,170,947 Habgood Aug. 29. 1939 FOREIGN PATENTS Number Country Date a 481,396 Great Britain Mar. 10, 1938 OTHER REFERENCES Ind. Eng. Chem, vol. 31 of 1939,,pages 950-956. 

